The macrophage is a relatively large, phagocytic cell. They play an essential role
in many different types of immune and inflammatory reactions. Macrophages
have multiple functions. They are important in killing intracellular parasites and
tumor cells. They act as scavengers for foreign material and extracellular debris.
They also act as regulators of immune responsiveness.
The major functional roles of macrophages in the immunological process are
antigen processing and antigen presentation.
An additional function attributed to macrophages is the production of factors that
influence the activity of lymphocytes. Macrophages secrete over 50 products,
many related to immunity. These include enzymes, plasma proteins (including
coagulation proteins and complement components), lipids, and factors regulating
cellular functions. One of the factors regulating cellular functions is interleukin-1,
which has a number of important effects. For example, interleukin-1, also called
lymphocyte-activating factor (LAF), induces lymphocytes to produce interleukin-2,
which in turn encourages short-term proliferation of lymphocytes.
Antigen bound to macrophage surfaces or internalized by macrophages is more
immunogenic than antigen that has not been "processed" by macrophages.
Macrophages function in processing the antigen and subsequently presenting it to
lymphocytes. It is thought that processing may expose determinants otherwise not
available or change pre-existing determinants into a recognizable form.
Once the antigen is processed by the macrophage, it is presented to lymphocytes.
There is evidence that small amounts of antigen bound to the macrophage surface
are important in the induction phase of the immune response. Evidence also
suggests that macrophage processing is not essential for all antigens. The size of
the antigen may determine whether macrophage processing is necessary.
A common opinion about the interaction of macrophages with B and T cells is that
macrophages digest complex antigens to make them "palatable" for B cells.
Another concept is that macrophages ingest antigens and then manufacture some
informational type of ribonucleic acid which is transferred to B lymphocytes and
triggers antibody production.